1. Technical Field
The present invention is directed to a method and apparatus for characterization of a thermal response of giant magnetoresistive (GMR) sensors in magnetic heads for disk drives.
2. Description of Related Art
The requirement of high density magnetic storage of data on hard disk drives has been increasing steadily for the past several years. Hard disk drives include magnetic heads for reading and writing data to the hard disk. The magnetic heads include write coils and sensors for reading data from the disks.
Development of magnetoresistive (MR) sensors (also referred to as heads) for disk drives in the early 1990's allowed disk drive products to maximize storage capacity with a minimum number of components (heads and disks). Fewer components result in lower storage costs, higher reliability, and lower power requirements for the hard disk drives.
MR sensors are used for the read element of a read/write head on a high-density magnetic disk. MR sensors read magnetically encoded information from the magnetic medium of the disk by detecting magnetic flux stored in the magnetic medium of the disk. As storage capacity of disk drives has increased, the storage bit has gotten smaller and its magnetic field has correspondingly become weaker. MR heads are more sensitive to weaker magnetic fields than are the inductive read coils used in earlier disk drives. Thus, there has been a move away from inductive read coils to MR sensors for use in disk drives.
During operation of the hard disk drive, sense current is passed through the MR element of the sensor causing a voltage drop. The magnitude of the voltage drop is a function of the resistance of the MR element. Resistance of the MR element varies in the presence of a magnetic field. Therefore, as the magnitude of the magnetic field flux passing through the MR element varies, the voltage across the MR element also varies. Differences in the magnitude of the magnetic flux entering the MR sensor can be detected by monitoring the voltage across the MR element.
As discussed above, MR sensors are known to be useful in reading data with a sensitivity exceeding that of inductive or other thin film sensors. However, the development of Giant Magnetoresistive (GMR) sensors (also referred to as GMR head chips) has greatly increased the sensitivity and the ability to read densely packed data. Thus, although the storage density for MR disks is expected to eventually reach 5 gigabits per square inch of surface disk drive (Gbits/sq.in.), the storage density of GMR disks is expected to exceed 100 Gbits/sq.in.
The GMR effect utilizes a spacer layer of a non-magnetic metal between two magnetic metals. The non-magnetic metal is chosen to ensure that coupling between magnetic layers is weak. GMR disk drive sensors (or head chips) operate at low magnetic layers. When the magnetic alignment of the magnetic layers is parallel, the overall resistance is relatively low. When the magnetic alignment of the layers is anti-parallel, the overall resistance is relatively high. When the sensor is biased with a constant current source, the change in resistance results in a change of voltage (“signal voltage”) across the GMR sensor. For a given GMR technology, the signal voltage is proportional to the amount of current passed through the GMR sensor. The current passing through the GMR sensor affects the temperature of the GMR sensor and thus, the thermal noise voltages. Large currents result in significant temperature change, and a large increase in the noise voltages. As the temperature increases, the ratio of signal voltage to noise voltage is reduced. This signal to noise ratio determines the bandwidth achievable by the GMR sensor.
Because GMR sensors allow extremely high data densities on disk drives, a stable sensor is essential to accurate read and write operations in high track density hard disk drives. It is known that temperature increases may cause the GMR sensor within the GMR element to exhibit unstable magnetic properties and efforts to reduce the temperature within the disk drive are ongoing.
As the requirements for the GMR sensors have been increasing over the years, the requirements for the write coils within the disk drives have also been increasing. New disk drives require fast field reversal during the write operation. This requirement for fast field reversal during the write operation implies larger write currents for gigahertz operation. Also, as the storage densities increase, the media coercivity has to increase to avoid thermal instability and the superparamagnetic limit. This also implies that larger write currents are necessary. However, large write currents increase the Joule heating in the coil such that the coil temperatures are commonly 40 to 80 degrees Celsius above ambient temperatures. However, for optimal operation, the write coils need to be kept near ambient temperatures.
Several passive and active cooling methods have been proposed to reduce the temperatures in the magnetic heads. These methods and designs require accurate determination of the thermal conductivity and/or microscale temperature characterization. The traditional thermal characterization methods cannot be easily extended to microscopic characterization because of increased parasitic losses associated with the magnetic head probes.
Scanning thermal microscopy (SThM) is a promising technique for microscale thermal characterization and has been recently used for this application. Unfortunately, the SThM methods proposed do not yield accurate temperature profiles. There is a substantial drop in temperature between the probe tip and the GMR/dielectric surface due to interface impedance. Hence it is difficult to calculate the thermal conductivities of these low thermal conductivity thin film materials.
Thus, there is a need for a mechanism by which thermal conductivities of low thermal conductivity thin film materials can be accurately calculated for use in thermal management of magnetic read/write heads.